a) Field of the Invention
This invention relates to a hydraulic control system for a construction machine such as a hydraulic excavator, and especially to a hydraulic control system for controlling a delivery rate of a variable displacement hydraulic pump by a positive control method.
b) Description of the Related Art
As a conventional hydraulic control system for an actuator arranged in a construction machine, a positive control system that controls a delivery rate of a pump by a pilot pressure outputted from a pilot control valve is known to the public.
FIG. 7 is a hydraulic circuit diagram illustrating a conventional hydraulic control system of such a positive control type for a construction machine, FIG. 8 is a diagram showing output characteristics of a pilot control valve disposed in the hydraulic control system depicted in FIG. 7, and FIG. 9 is a diagram showing delivery rate characteristics of a variable displacement hydraulic pump arranged in the hydraulic control system depicted in FIG. 7.
The conventional hydraulic control system shown in FIG. 7 is provided with a variable displacement hydraulic pump 1 making up a main pump, an actuator driven by pressure fluid delivered from the variable displacement hydraulic pump 1, for example, a hydraulic cylinder 4, and a directional control valve 3 for controlling a flow of the pressure fluid to be fed from the variable displacement hydraulic pump 1 to the hydraulic cylinder 4. When the directional control valve 3 is switched to a position 3a, the hydraulic cylinder 4 is brought, at a bottom side thereof, into communication with the variable displacement hydraulic pump 1 and is also brought, at a rod side thereof into communication with a tank 9. When the directional control valve 3 is switched to a position 3b, on the other hand, the hydraulic cylinder 4 is brought, at the rod side thereof, into communication with the variable displacement hydraulic pump 1 and is also brought, at the bottom side thereof, into communication with the tank 9. The delivery rate of the variable displacement hydraulic pump 1 is controlled by a flow control unit 2. The hydraulic cylinder 4 drives a working equipment of a work vehicle which is not illustrated. If the work vehicle is a hydraulic excavator, for example, the hydraulic cylinder 4 can be an arm cylinder, a boom cylinder or the like.
The conventional hydraulic control system shown in FIG. 7 is also equipped with a pilot control valve 5 for outputting a pilot pressure Pa adapted to switch the directional control valve 3, a pilot pump 7 for feeding pressure fluid to the pilot control valve 5, and a pilot relief valve 8 for specifying the delivery pressure of the pilot pump 7. The pilot control valve 5 is composed of pressure-reducing valve portions 10, 11 and a control lever 5a for operating these pressure-reducing valve portions 10, 11. The pressure-reducing valve portion 10 is connected to one of drive control units of the directional control valve 3 via a pilot line 13, whereas the pressure-reducing valve portion 11 is connected to the other drive control unit of the directional control valve 3 via a pilot line 14. The conventional hydraulic control system is also equipped with a selector valve 6, which selects one of a greater value out of a pilot pressure Pa in the pilot line 13 and another pilot pressure Pa in the pilot line 14 and outputs the pilot pressure of the greater value as a pump control signal Pc to the flow control unit 2 via a signal line 12.
Output characteristics of the pilot control valve 5, that is, a relationship between a stroke S of the control lever 5a and pilot pressures Pa outputted from the pressure-reducing valve portions 10, 11 is set in such a way that, as is illustrated in FIG. 8, each pilot pressure Pa is outputted at a level substantially proportional with the stroke S. Further, delivery rate characteristics of the variable displacement hydraulic pump 1, that is, a relationship between the pump control signal Pc applied to the flow control unit 2 and a pump flow rate Q delivered from the variable displacement hydraulic pump 1 is set in such a way that, as is shown in FIG. 9, the pressure fluid is delivered at a pump flow rate Q which is substantially in proportion with the pump control signal Pc (which is equal to the above-selected pilot pressure Pa).
According to the conventional hydraulic control system constructed as described above, turning of the control lever 5a of the pilot control valve 5 over a stroke S in the direction of arrow 5b, for example, to make the hydraulic cylinder 4 perform an extending operation actuates the pressure-reducing valve 10, and a pilot pressure Pa is outputted from the pressure-reducing valve portion 10. This pilot pressure Pa is outputted to the one drive control unit of the directional control valve 3 via the pilot line 13 so that the directional control valve 3 is switched to the position 3a. At this time, the pilot pressure Pa in the pilot line 13 is selected by the selector valve 6, and this pilot pressure Pa is outputted as a pump control signal Pc to the signal line 12. The flow control unit 2 is driven by the pump control signal Pc, and the pump flow rate Q of the pressure fluid delivered from the variable displacement hydraulic pump 1 is controlled at a level corresponding to the stroke S of the control lever 5a of the pilot control valve 5. The pressure fluid delivered at the pump flow rate Q from the variable displacement hydraulic pump 1 is fed to the bottom side of the hydraulic cylinder 4 via the directional control valve 3, and the fluid on the rod side is returned to the tank 9 via the directional control valve 3. As a consequence, the hydraulic cylinder 4 performs an extending operation so that the associated working equipment of the unillustrated work vehicle is driven.
As conventional art of this kind, reference may be had to Japanese Patent No. 2,534,897.
In the above-described conventional hydraulic control system shown in FIG. 7, the pilot pressure Pa of the greater value fed to the selector valve 6 is guided, as is, as the pump control signal Pc to the signal line 12 to drive the flow control unit 2. Due to a design limitation to the layout that the position of a pilot circuit--which includes the pilot control valve 5, the directional control valve 3, the pilot lines 13, 14 and the selector valve 6--and the position of the flow control unit 2--which is arranged in the vicinity of the variable displacement hydraulic pump 1--have to be separated sufficiently from each other, the above-mentioned signal line 12 is generally made of a rubber hose, the length of which is as long as several meters. Accordingly, the total volume of the volume of the signal line 12 and the volumes of the pilot lines 13, 14 connected to the respective drive control units of the directional control valve 3 is relatively large.
In the conventional hydraulic control system shown in FIG. 7, it therefore takes some time after operation of the control lever 5a and hence actuation of the pilot control valve 5 until the cumulative quantity of the pilot pressure fluid outputted from the pressure-reducing valve portion 10 or the pressure-reducing valve portion 11 becomes equal to the above-mentioned total volume, although this time is not substantial. As a result, the switching of the directional control valve 3 tends to be delayed. Further, an increase in the flow rate of pressure fluid to the line 13 or 14 and thus to the line 12 results in the occurrence of a greater pressure loss at the selector valve 6, so that the actuation of the flow control unit 2 also tends to become late. In other words, the switching responsibility of the directional control valve 3 to the operation of the pilot control valve 5 tends to be reduced and, moreover, the follow-up controllability of an increase or decrease in the pump flow rate Q of the variable displacement hydraulic pump 1 for the operation of the pilot control valve 5 also tends to be deteriorated. For these, the conventional hydraulic control system is accompanied by problems that the accuracy of driving control of the hydraulic cylinder 4 controlled by the directional control valve 3, that is, of the actuator tends to be lowered and no improvement can thus be expected in the efficiency of work to be performed by the construction machines